Onyx is a cryptocrystalline form of quartz. The colors of its
bands range from white to almost every color (save some shades, such as purple
or blue). Commonly, specimens of onyx available contain bands of colors of
white, tan, and brown. Sardonyx is a variant in which the colored bands are sard
(shades of red) rather than black. Pure black onyx is common, and perhaps the
most famous variety, but not as common as onyx with banded colors.